Fix Travis failures (#2957)

* Tweak .travis.yml

* Use R_REMOTES_NO_ERRORS_FROM_WARNINGS=true

c.f. travis-ci/travis-ci#10285 (comment)

Author: yutannihilation

.travis.yml

@@ -9,15 +9,12 @@ matrix:
   - r: devel
   - r: release
     env: USE_VDIFFR=true
+    r_github_packages: lionel-/vdiffr
   - r: oldrel
   - r: 3.2
+    env: R_REMOTES_NO_ERRORS_FROM_WARNINGS=true
   - r: 3.1
-
-# work around temporary travis + R 3.5 bug
-r_packages: devtools
-
-# Install manually because dev vdiffr is not compatible with Appveyor yet
-r_github_packages: lionel-/vdiffr
+    env: R_REMOTES_NO_ERRORS_FROM_WARNINGS=true
 
 # environment variables set for all builds
 env: